m (Ja Purity resize header moved to Customising the JA Purity template/customisations/Resizing the header: Moved page to a subpage of Customising the JA Purity template/customisations for consistency)
The original JA Purity template was provided from JoomlArt.com as a zip but the latest files have been installed along with Joomla.
The relevant file, template.css is in the following directory:
templates/ja_purity css/ template.css
If you've been following the tutorial you can download a tutorial version of the template that installs to the following directory:
templates/my_japurity css/ template.css
This article will help you alter the height of the header.
You can edit the CSS file directly in your choice of plain text editors such as VI or Notepad or you can use the CSS Editor provided in the Joomla Administration area.
If you prefer to keep the current size of your logo you may want to change it's position.